Hot-dip galvanized steel sheet and manufacturing method therefor

ABSTRACT

Provided is a hot-dip galvanized steel sheet having both high strength and good workability, as well as excellent coating quality. The chemical composition of the base steel sheet is set within a specified range, the steel microstructure of the base steel sheet is a complex structure of ferrite, martensite and bainite, oxygen is present as oxides in the surface layer of the base steel sheet in an amount of 0.05 g/m 2  or more and 0.50 g/m 2  or less per surface, and the hot-dip galvanized layer contains Fe in an amount of 0.40 mass % or more.

TECHNICAL FIELD

This disclosure relates to a hot-dip galvanized steel sheet that is suitably used for automobile members and the like, and a method of manufacturing the hot-dip galvanized steel sheet.

BACKGROUND

In recent years, enhancement of fuel efficiency of automobiles has become an important issue from the viewpoint of global environment protection. Therefore, there is a growing trend to increase the strength and reduce the thickness of steel sheets used as materials for automobile members to reduce the weight of automotive bodies. Further, the steel sheets used for automobile members are formed into complicated shapes, so that they are required to have good workability.

In response to such a request, for example, JP 2012-172159 A (PTL 1) describes “a high-strength cold-rolled steel sheet with excellent uniform deformability and local deformability, which contains, in mass %,

-   -   C: 0.01% or more and 0.4% or less,     -   Si: 0.001% or more and 2.5% or less,     -   Mn: 0.001% or more and 4.0% or less,     -   P: 0.001% or more and 0.15% or less,     -   S: 0.0005% or more and 0.03% or less,     -   Al: 0.001% or more and 2.0% or less,     -   N: 0.0005% or more and 0.01% or less, and     -   O: 0.0005% or more and 0.01% or less,         with the balance being iron and inevitable impurities, wherein         the steel sheet comprises a texture where the average value of         the X-ray random intensity ratios of the {112}<110> to         {113}<110> orientation group and the {112}<131> crystal         orientation of at least the sheet surface at ⅝ to ⅜ sheet         thickness from the surface of the steel sheet is 5.0 or less,         the X-ray random intensity ratio of the {001}<110> crystal         orientation is 4.0 or less, the r (rC) value in a direction         orthogonal to the rolling direction is 0.70 or more, and the r         value at 300 (r30) from the rolling direction is 1.10 or less,         and a microstructure where the total area ratio of ferrite and         bainite is 50% or more, and the area ratio of martensite is 1%         or more and 50% or less”.

Further, J P 2009-249733A (PTL 2) describes “a high-strength steel sheet having excellent hardenability with very little aging deterioration, which contains, in mass %,

-   -   C: 0.05% to 0.20%,     -   Si: 0.3% to 1.50%,     -   Mn: 1.3% to 2.6%,     -   P: 0.001% to 0.03%,     -   S: 0.0001% to 0.01%,     -   Al: 0.0005% to 0.1%,     -   N: 0.0005% to 0.0040%, and     -   O: 0.0015% to 0.007%,         with the balance being iron and inevitable impurities, wherein         the steel sheet microstructure is mainly composed of ferrite and         bainite, the BH after baking treatment is 60 MPa or more, and         the maximum tensile strength is 540 MPa or more.”

CITATION LIST Patent Literature

-   PTL 1: JP 2012-172159 A -   PTL 2: JP 2009-249733A

SUMMARY Technical Problem

However, from the viewpoint of rust resistance of automobile bodies, steel sheets used as materials for automobile members are sometimes subjected to zinc or zinc alloy coating or plating, such as hot-dip galvanizing.

However, when the steel sheets described in PTLs 1 and 2 are subjected to hot-dip galvanizing, the coating or plating quality such as coating or plating appearance and coating or plating adhesion may be insufficient. Therefore, it is desired to make improvement in this regard.

It could thus be helpful to provide a hot-dip galvanized steel sheet that has both high strength and good workability, as well as excellent coating quality.

It is also helpful to provide a method of manufacturing the hot-dip galvanized steel sheet.

Solution to Problem

As a result of intensive studies, we discovered the following.

(a) To obtain good workability, it is necessary to improve the hole expansion formability and the elongation of a steel sheet. From the viewpoint of preventing cracking during forming, it is effective to increase the yield ratio YR (=yield stress (YS)/tensile strength (TS)) of a steel sheet.

(b) To obtain high strength, it is effective to use martensite. On the other hand, the use of ferrite is effective in obtaining excellent elongation. Further, to obtain excellent hole expansion formability, it is necessary to reduce the hardness difference between ferrite, which is a soft phase, and martensite, which is a hard phase. This can be effectively achieved by using bainite, which is an intermediate phase. Further, the use of bainite increases the yield ratio.

(c) That is, when the steel microstructure is a complex structure in which ferrite, martensite and bainite are controlled to predetermined area ratios (hereinafter referred to simply as “complex structure”), it is possible to achieve both high strength and good workability.

(d) Furthermore, to obtain good coating or plating quality, it is effective to

-   -   cause internal oxidation in a surface layer of a base steel         sheet before coating or plating treatment to form oxides of Si         and Mn in the surface layer of the base steel sheet, and     -   contain an appropriate amount of Fe in a hot-dip galvanized         layer.

That is, it is effective to use Si and Mn in terms of increasing the strength of a steel sheet. However, elements such as Si and Mn are oxidizable elements, which combine with oxygen to form oxides on the steel sheet surface. The presence of such Si and Mn oxides on the surface of the base steel sheet during the coating or plating treatment reduces the wettability of the base steel sheet by a coating or plating bath (hot dip zinc), causing poor coating or plating appearance such as non-coating or non-plating and deterioration of coating or plating adhesion.

In this regard, if internal oxidation is caused in the surface layer of the base steel sheet to form oxides of Si and Mn before the coating or plating treatment, these oxides in the surface layer of the base steel sheet serve as a barrier, and the formation of oxides on the surface of the base steel sheet (hereinafter referred to as external oxidation) is suppressed. As a result, the coating or plating quality such as coating or plating appearance and coating or plating adhesion is improved.

Further, the coating or plating quality, especially coating or plating adhesion, is improved by containing an appropriate amount of Fe in a hot-dip galvanized layer.

(e) In addition, it is important to properly control the annealing conditions prior to the coating or plating treatment and the coating or plating treatment conditions to create a complex structure as described above, to form oxides of Si and Mn in the surface layer of the base steel sheet by causing internal oxidation in the surface layer of the base steel sheet, and also to contain an appropriate amount of Fe in the hot-dip galvanized layer. It is particularly important to control the atmosphere during the holding of annealing and to control the temperature of the cold-rolled steel sheet when it enters the coating or plating bath in the coating or plating treatment.

Specifically, when the dew point is set in a range of −20° C. or higher and 5° C. or lower and a certain amount of oxygen is ensured in the holding atmosphere of annealing, the internal oxidation in the surface layer of the base steel sheet is promoted. On the other hand, when the hydrogen concentration is set to 3 mass % or more and 20 mass % or less, oxides that have been formed on the surface of the base steel sheet (and oxides that have been formed during the holding of annealing) are reduced. Therefore, it is important to suppress the external oxidation while introducing sufficient oxygen from the atmosphere into the interior (surface layer) of the base steel sheet. It is also important to promote the diffusion of Fe from the base steel sheet to the coated or plated layer by setting the temperature of the cold-rolled steel sheet when it enters the coating or plating bath to at least 10° C. higher than the coating or plating bath temperature.

The present disclosure is based on these discoveries and further studies.

We thus provide the following.

[1] A hot-dip galvanized steel sheet comprising a base steel sheet and a hot-dip galvanized layer on a surface of the base steel sheet, wherein

-   -   the base steel sheet comprises     -   a chemical composition containing (consisting of), in mass %,         -   C: 0.09% or more and 0.17% or less,         -   Si: 0.3% or more and 1.1% or less,         -   Mn: 1.9% or more and 2.7% or less,         -   P: 0.10% or less,         -   S: 0.050% or less,         -   Al: 0.01% or more and 0.20% or less, and         -   N: 0.10% or less,         -   with the balance being Fe and inevitable impurities, and a             steel microstructure where         -   ferrite has an area ratio of 30% or more and 85% or less,         -   martensite has an area ratio of 5% or more and 30% or less,         -   bainite has an area ratio of 10% or more and 60% or less,             and         -   other metallic phases have an area ratio of 15% or less         -   with respect to the entire steel microstructure,     -   oxygen is present as oxides in a surface layer of the base steel         sheet in an amount of 0.05 g/m² or more and 0.50 g/m² or less         per surface, where the surface layer is an area from a surface         of the base steel sheet to a position at a depth of 100 μm, and     -   the hot-dip galvanized layer contains Fe in an amount of 0.40         mass % or more.

[2] The hot-dip galvanized steel sheet according to aspect [1], wherein the other metallic phases have an area ratio of 5% or less.

[3] The hot-dip galvanized steel sheet according to aspect [1] or [2], wherein the hot-dip galvanized layer contains Fe in an amount of 8.0 mass % or less.

[4] The hot-dip galvanized steel sheet according to any one of aspects [1] to [3], wherein the hot-dip galvanized layer has a coating weight of 20 g/m² or more per surface.

[5] The hot-dip galvanized steel sheet according to any one of aspects [1] to [4], wherein the chemical composition of the base steel sheet further contains, in mass %, at least one selected from the group consisting of

-   -   Nb: 0.040% or less,     -   Ti: 0.030% or less,     -   B: 0.0030% or less,     -   Cr: 0.3% or less,     -   Mo: 0.2% or less, and     -   V: 0.065% or less.

[6] The hot-dip galvanized steel sheet according to any one of aspects [1] to [5], wherein the chemical composition of the base steel sheet further contains, in mass %,

-   -   at least one selected from the group consisting of Ta, W, Ni,         Cu, Sn, Sb, Ca, Mg and Zr in a total amount of 0.1% or less.

[7] A method of manufacturing a hot-dip galvanized steel sheet, comprising:

-   -   a hot rolling process where a steel slab having the chemical         composition as recited in any one of aspects [1], [5], or [6] is         subjected to hot rolling to obtain a hot-rolled steel sheet,     -   a cold rolling process where the hot-rolled steel sheet is         subjected to cold rolling to obtain a cold-rolled steel sheet,     -   an annealing process where the cold-rolled steel sheet is heated         to an annealing temperature, held at the annealing temperature,         and then cooled, and     -   then a coating treatment process where the cold-rolled steel         sheet is subjected to hot-dip galvanizing treatment, wherein     -   in the annealing process,         -   an average heating rate in a temperature range from 500° C.             to the annealing temperature is 1° C./s or higher and 7°             C./s or lower,         -   the annealing temperature is (A_(C1) point+50° C.) or higher             and (A_(C3) point+20° C.) or lower,         -   during the holding, a holding time is 1 second or longer and             40 seconds or shorter,         -   during the holding, an atmosphere has a dew point of −20° C.             or higher and 5° C. or lower, and a hydrogen concentration             is 3 mass % or more and 20 mass % or less,         -   an average cooling rate in a temperature range from the             annealing temperature to a primary cooling stop temperature             is 10° C./s or higher,         -   the primary cooling stop temperature is 450° C. or higher             and 600° C. or lower,         -   a secondary cooling time is 20 seconds or longer and 100             seconds or shorter, and         -   a secondary cooling stop temperature is 400° C. or higher             and 500° C. or lower, and     -   in the coating treatment process,         -   a temperature of the cold-rolled steel sheet when it enters             a coating bath is at least 10° C. higher than a coating bath             temperature.

Advantageous Effect

According to the present disclosure, it is possible to obtain a hot-dip galvanized steel sheet having both high strength and good workability, as well as excellent coating quality.

By applying the hot-dip galvanized steel sheet of the present disclosure to automobile members, the performance of automobile bodies can be significantly improved.

DETAILED DESCRIPTION

The present disclosure will be described based on the following embodiments.

First, the chemical composition of a base steel sheet of a hot-dip galvanized steel sheet according to one embodiment of the present disclosure will be described. The “%” representations below indicating the chemical composition are in “mass %” unless stated otherwise.

C: 0.09% or More and 0.17% or Less

C is an element that improves the hardenability. C also plays a role in increasing the strength of ferrite. Therefore, it is required to contain C to ensure a desired tensile strength (TS) of 750 MPa or more. When the C content is less than 0.09%, the desired tensile strength cannot be obtained. Therefore, the C content is set to 0.09% or more. The C content is preferably 0.10% or more and more preferably 0.11% or more. On the other hand, if the C content exceeds 0.17%, the stability of austenite increases, and it is difficult to form bainite. In addition, the strength of martensite increases excessively, and the yield ratio decreases. Therefore, the C content is set to 0.17% or less. The C content is preferably 0.16% or less and more preferably 0.15% or less.

Si: 0.3% or More and 1.1% or Less

Si is a solid-solution-strengthening element. Si also plays a role in increasing the yield ratio by increasing the strength of ferrite. To obtain this effect, the Si content is set to 0.3% or more. The Si content is preferably 0.4% or more and more preferably 0.5% or more. On the other hand, if the Si content is too high, Si concentrates on the surface of the base steel sheet, causing external oxidation and deteriorating the coating quality such as coating appearance. Therefore, the Si content is set to 1.1% or less. The Si content is preferably 1.0% or less and more preferably 0.9% or less.

Mn: 1.9% or More and 2.7% or Less

Mn is an element that improves the hardenability of steel. Therefore, it is required to contain Mn to ensure the desired tensile strength. When the Mn content is less than 1.9%, the desired tensile strength cannot be obtained. Therefore, the Mn content is set to 1.9% or more. The Mn content is preferably 2.0% or more and more preferably 2.1% or more. On the other hand, if the Mn content is too high, Mn concentrates on the surface of the base steel sheet, causing external oxidation and deteriorating the coating quality such as coating appearance. In addition, Mn tends to concentrate into austenite during, for example, the holding of annealing, and the strength of martensite that transforms from austenite excessively increases. Therefore, the Mn content is set to 2.7% or less. The Mn content is preferably 2.6% or less and more preferably 2.5% or less.

P: 0.10% or Less

P is an element that strengthens steel. However, if the P content is too high, P segregates to grain boundaries and deteriorates the hole expansion formability. Therefore, the P content is set to 0.10% or less. The P content is preferably 0.05% or less and more preferably 0.03% or less. Although the lower limit of the P content is not particularly limited, it is preferably 0.001% or more from the viewpoint of cost, for example. The P content is more preferably 0.003% or more and even more preferably 0.005% or more.

S: 0.050% or Less

S is an element that deteriorates the elongation through the formation of MnS and the like. If Ti is contained together with S, the hole expansion formability may be deteriorated due to the formation of, for example, TiS and Ti(C,S). Therefore, the S content is set to 0.050% or less. The S content is preferably 0.030% or less, more preferably 0.020% or less, and even more preferably 0.015% or less. Although the lower limit of the S content is not particularly limited, it is preferably 0.0002% or more from the viewpoint of cost, for example. The S content is more preferably 0.0005% or more.

Al: 0.01% or More and 0.20% or Less

Al is an element added as a deoxidizing material. Al also plays a role in reducing coarse inclusions in the steel and improving the hole expansion formability. When the Al content is less than 0.01%, the above effect is insufficient. Therefore, the Al content is set to 0.01% or more. The Al content is preferably 0.02% or more. On the other hand, if the Al content exceeds 0.20%, nitride-based precipitates such as AlN are coarsened, and the hole expansion formability is deteriorated. Therefore, the Al content is set to 0.20% or less. The Al content is preferably 0.17% or less and more preferably 0.15% or less.

N: 0.10% or Less

N is an element that contributes to the improvement of hole expansion formability by forming nitride-based precipitates such as AlN that pin crystal grain boundaries. However, if the N content exceeds 0.10%, nitride-based precipitates such as AlN are coarsened, and the hole expansion formability is deteriorated. Therefore, the N content is set to 0.10% or less. The N content is preferably 0.05% or less and more preferably 0.010% or less. Although the lower limit of the N content is not particularly limited, it is preferably 0.0006% or more from the viewpoint of cost, for example. The N content is more preferably 0.0010% or more.

The base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present disclosure has a chemical composition containing the above elements and the balance of Fe (iron) and inevitable impurities. It is particularly preferable that the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present disclosure has a chemical composition containing the above elements, with the balance consisting of Fe and inevitable impurities.

The above describes the basic chemical composition of the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present disclosure. The base steel sheet may contain, as optional elements, at least one selected from the group consisting of

-   -   Nb: 0.040% or less,     -   Ti: 0.030% or less,     -   B: 0.0030% or less,     -   Cr: 0.3% or less,     -   Mo: 0.2% or less, and     -   V: 0.065% or less.

Further, it may contain, as optional elements, at least one selected from the group consisting of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg, and Zr, where the selected elements are contained in a total amount of 0.1% or less.

If any of the above optional elements is contained in an amount less than the suitable lower limit described below, this element is regarded as an inevitable impurity.

Nb: 0.040% or Less

Nb contributes to increasing the strength through the refinement of prior γ grains and the formation of fine precipitates. In addition, the fine precipitates increase the strength of ferrite and contribute to increasing the yield ratio. To obtain this effect, the Nb content is preferably 0.0010% or more. The Nb content is more preferably 0.0015% or more and even more preferably 0.0020% or more. On the other hand, an excessively high Nb content results in an excessive amount of carbonitride-based precipitate, which deteriorates the hole expansion formability. Therefore, when Nb is contained, the Nb content is preferably 0.040% or less. The Nb content is more preferably 0.035% or less and even more preferably 0.030% or less.

Ti: 0.030% or Less

Ti, like Nb, contributes to increasing the strength through the refinement of prior γ grains and the formation of fine precipitates. In addition, the fine precipitates increase the strength of ferrite and contribute to increasing the yield ratio. To obtain this effect, the Ti content is preferably 0.0010% or more. The Ti content is more preferably 0.0015% or more and even more preferably 0.0020% or more. On the other hand, an excessively high Ti content results in an excessive amount of carbonitride-based precipitate, which deteriorates the hole expansion formability. Therefore, when Ti is contained, the Ti content is preferably 0.030% or less. The Ti content is more preferably 0.025% or less and even more preferably 0.020% or less.

B: 0.0030% or Less

B is an element that improves the hardenability of steel. The inclusion of B renders it possible to achieve the desired tensile strength even when the Mn content is low. To obtain this effect, the B content is preferably 0.0001% or more. The B content is more preferably 0.0002% or more. On the other hand, a B content of 0.0030% or more results in an excessive amount of nitride-based precipitate such as BN, which deteriorates the hole expansion formability. Therefore, when B is contained, the B content is preferably 0.0030% or less. The B content is more preferably 0.0025% or less and even more preferably 0.0020% or less.

Cr: 0.3% or Less

Cr is an element that improves the hardenability of steel. To obtain this effect, the Cr content is preferably 0.005% or more. However, an excessively high Cr content may cause oxide formation reaction accompanied by the formation of hydrogen ions, which may deteriorate the coating quality. Further, precipitates such as carbides are excessively precipitated, and the hole expansion formability is deteriorated. Therefore, when Cr is contained, the Cr content is preferably 0.3% or less. The Cr content is more preferably 0.2% or less and even more preferably 0.1% or less.

Mo: 0.2% or Less

Mo, like Cr, is an element that improves the hardenability of steel. To obtain this effect, the Mo content is preferably 0.005% or more. However, an excessively high Mo content may cause oxide formation reaction accompanied by the formation of hydrogen ions, which may deteriorate the coating quality. Further, precipitates such as carbides are excessively precipitated, and the hole expansion formability is deteriorated. Therefore, when Mo is contained, the Mo content is preferably 0.2% or less. The Mo content is more preferably 0.1% or less and even more preferably 0.04% or less.

V: 0.065% or Less.

V, like Cr, is an element that improves the hardenability of steel. To obtain this effect, the V content is preferably 0.005% or more. However, an excessively high V content may cause oxide formation reaction accompanied by the formation of hydrogen ions, which may deteriorate the coating quality. Further, precipitates such as carbides are excessively precipitated, and the hole expansion formability is deteriorated. Therefore, when V is contained, the V content is preferably 0.065% or less. The V content is more preferably 0.050% or less and even more preferably 0.035% or less.

At least one selected from the group consisting of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr in a total amount of 0.1% or less.

Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr are elements that increase the strength without deteriorating the coating quality. To obtain this effect, the content of these elements is preferably 0.0010% or more, either singly or in total. However, when the total content of these elements exceeds 0.1%, the above effect is saturated. Therefore, when at least one selected from the group consisting of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg, and Zr are contained, the total content of these elements is preferably 0.1% or less.

The balance other than the aforementioned elements is Fe and inevitable impurities.

Next, the steel microstructure of the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present disclosure will be described.

The steel microstructure of the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present disclosure is a complex structure where

-   -   ferrite has an area ratio of 30% or more and 85% or less,     -   martensite has an area ratio of 5% or more and 30% or less, and     -   bainite has an area ratio of 10% or more and 60% or less         with respect to the entire steel microstructure. Note that the         area ratio refers to a ratio of the area of each metallic phase         to the area of the entire steel microstructure.

Area Ratio of Ferrite: 30% or More and 85% or Less

Ferrite is a necessary phase from the viewpoint of obtaining desired elongation. Therefore, the area ratio of ferrite is set to 30% or more. The area ratio of ferrite is preferably 35% or more and more preferably 40% or more. On the other hand, an excess of ferrite reduces the area ratio of martensite required to ensure the strength, rendering it difficult to ensure the strength. It also suppresses the formation of bainite and reduces the hole expansion formability and the yield ratio. Therefore, the area ratio of ferrite is set to 85% or less. The area ratio of ferrite is preferably 80% or less.

As used herein, the ferrite is a microstructure containing crystal grains of BCC lattice, which is formed by transformation from austenite at relatively high temperatures.

Area Ratio of Martensite: 5% or More and 30% or Less

Martensite contributes to the improvement of strength and is a phase necessary for ensuring the desired tensile strength. Therefore, the area ratio of martensite is set to 5% or more. The area ratio of martensite is preferably 8% or more and more preferably 10% or more. On the other hand, an excess of martensite deteriorates the elongation. Therefore, the area ratio of martensite is set to 30% or less. The area ratio of martensite is preferably 28% or less and more preferably 25% or less.

As used herein, the martensite refers to a hard microstructure formed from austenite at or below the martensite transformation temperature (also referred to simply as “Ms point”), which includes both so-called fresh martensite as quenched and so-called tempered martensite where fresh martensite is reheated and tempered.

Area Ratio of Bainite: 10% or More and 60% or Less

Bainite is a phase necessary for improving the hole expansion formability and increasing the yield ratio. Therefore, the area ratio of bainite is set to 10% or more. The area ratio of bainite is preferably 15% or more and more preferably 20% or more. On the other hand, an excess of bainite deteriorates the elongation. Therefore, the area ratio of bainite is set to 60% or less. The area ratio of bainite is preferably 55% or less and more preferably 50% or less.

As used herein, the bainite is a hard microstructure in which fine carbides are dispersed in needle-like or plate-like ferrite, and it is formed from austenite at relatively low temperatures (at or above the martensitic transformation temperature).

Area Ratio of Other Metallic Phases: 15% or Less

The steel microstructure of the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present disclosure may contain metallic phases other than martensite, ferrite, and bainite. It is acceptable if the total area ratio of other metallic phases is 15% or less. Therefore, the area ratio of other metallic phases is set to 15% or less. The area ratio of other metallic phases is preferably 10% or less and more preferably 5% or less. The area ratio of other metallic phases may be 0%.

Examples of the other metallic phases include pearlite, retained austenite, and non-recrystallized ferrite. Among these phases, pearlite and non-recrystallized ferrite deteriorate the workability (El and λ), so that the total area ratio of pearlite and non-recrystallized ferrite is set to 5% or less. The area ratios of pearlite and non-recrystallized ferrite may each be 0%. Because retained austenite does not deteriorate the workability (El and λ), there is no problem if the area ratio of retained austenite is 15% or less. The area ratio of retained austenite is preferably 10% or less and more preferably 5% or less. The area ratio of retained austenite may be 0% or less.

As used herein, the pearlite is a microstructure containing ferrite and needle-like cementite. The retained austenite is austenite remaining without being transformed into martensite. The non-recrystallized ferrite is ferrite that is not recrystallized, in which crystal grains include sub-boundaries.

As used herein, the area ratio of each phase is measured as follows.

A test piece is collected from the base steel sheet of the hot-dip galvanized steel sheet so that an L-section parallel to the rolling direction serves as a test surface. Next, the test surface of the test piece is subjected to mirror polishing, and the microstructure is revealed with a nital solution. The test surface of the test piece with the revealed microstructure is observed with a SEM at a magnification of 1500×, and the area ratio of martensite, the area ratio of ferrite, and the area ratio of bainite at the ¼ thickness position of the base steel sheet are measured with a point counting method.

In the SEM image, martensite is a white microstructure. Further, fine carbides are precipitated inside tempered martensite among the martensite. Ferrite is a black microstructure. Bainite has white carbides precipitated in a black microstructure. Each phase in the SEM image is identified based on the above description. However, depending on the plane orientation of block grains and the degree of etching, it may be difficult to reveal the internal carbides. In that case, etching is thoroughly performed for confirmation.

The total area ratio of the other metallic phases is calculated by subtracting the area ratio of martensite, the area ratio of ferrite, and the area ratio of bainite from 100%.

Among the other metallic phases, pearlite is a microstructure containing ferrite and needle-like cementite as described above. Based on this, pearlite is identified in the SEM image, and the area ratio of pearlite is measured. Non-recrystallized ferrite has sub-boundaries inside crystal grains as described above. Based on this, non-recrystallized ferrite is identified in the SEM image, and the area ratio of non-recrystallized ferrite is measured.

The Area Ratio of Retained Austenite is Measured as Follows.

The base steel sheet of the hot-dip galvanized steel sheet is polished in the thickness direction (depth direction) to the ¼ thickness position and then chemically polished by 0.1 mm to obtain an observation plane. Next, the observation plane is observed with the X-ray diffraction method. Using a Mo Kα source as an incident X-ray, ratios of the diffraction intensity of each of (200), (220) and (311) planes of fcc iron (austenite) to the diffraction intensity of each of (200), (211), and (220) planes of bcc iron are determined, and the volume fraction of retained austenite is calculated based on the ratio of diffraction intensity of each plane. Next, assuming that the retained austenite is three-dimensionally homogeneous, the volume fraction of retained austenite is taken as the area ratio of retained austenite.

Amount of oxygen present as oxide in the surface layer of the base steel sheet (hereinafter also referred to as “amount of oxygen in oxide form in the surface layer of the base steel sheet”): 0.05 g/m² or more and 0.50 g/m² or less per surface

As described above, it is effective to use Si and Mn in terms of increasing the strength of a steel sheet. However, elements such as Si and Mn are oxidizable elements, which combine with oxygen to form oxides on the steel sheet surface. The presence of such Si and Mn oxides on the surface of the base steel sheet during coating treatment reduces the wettability of the base steel sheet by a coating bath (hot-dip zinc), causing poor coating appearance such as non-coating and deterioration of coating adhesion.

In this regard, if internal oxidation is caused in the surface layer of the base steel sheet to form oxides of Si and Mn before the coating treatment, these oxides present in the surface layer of the base steel sheet serve as a barrier, and the formation of oxides on the surface of the base steel sheet (hereinafter referred to as “external oxidation”) is suppressed. As a result, the coating quality such as coating appearance and coating adhesion is improved. Therefore, the amount of oxygen in oxide form in the surface layer of the base steel sheet is set to 0.05 g/m² or more per surface (note that all the amount of oxygen described below is the amount for one surface). The amount of oxygen in oxide form in the surface layer of the base steel sheet is preferably 0.06 g/m² or more. On the other hand, if the amount of oxygen in oxide form in the surface layer of the base steel sheet exceeds 0.50 g/m², the oxides promote fracture and deteriorate the elongation and the hole expansion formability. Therefore, the amount of oxygen in oxide form in the surface layer of the base steel sheet is set to 0.50 g/m² or less. The amount of oxygen in oxide form in the surface layer of the base steel sheet is preferably 0.45 g/m² or less.

As used herein, the surface layer is an area from the surface of the base steel sheet to a position at a depth of 100 μm.

Oxides are compounds of oxygen and elements such as Si, Mn, Fe, P, Al, Nb, Ti, B, Cr, Mo, and V contained in the base steel sheet, and the oxides are mainly Si oxides and Mn oxides.

The amount of internal oxidation is inversely related to the amount of external oxidation. Therefore, if external oxidation occurs in the base steel sheet, the amount of oxygen in oxide form in the surface layer of the base steel sheet is less than 0.05 g/m².

The amount of oxygen in oxide form in the surface layer of the base steel sheet is measured with an “impulse furnace-infrared absorption method”.

First, the hot-dip galvanized layer is removed from the hot-dip galvanized steel sheet. The method of removing the hot-dip galvanized layer is not limited if the hot-dip galvanized layer can be totally removed. Examples thereof include pickling, alkali dissolution, and mechanical polishing.

Next, the amount of oxygen in the steel of the base steel sheet is measured. The measured value is taken as the total amount of oxygen OI (g) contained in the base steel sheet.

Next, at least the surface layers (an area from the surface of the base steel sheet to a position at a depth of 100 μm) on both sides of the base steel sheet is removed by polishing, and the amount of oxygen in the steel of the base steel sheet is measured after the surface layers have been removed. The measured value is taken as OH (g).

The amount of oxygen in oxide form in the surface layer of the base steel sheet is calculated based on the following formula.

[Amount of oxygen in oxide form in the surface layer of the base steel sheet]{OI(g)−OH(g)×([thickness of the base steel sheet before polishing(mm)]/[thickness of the base steel sheet after polishing(mm)])}÷([surface area of the base steel sheet(per surface)(m²)]÷2

In the above formula, the amount of oxygen in oxide form in the surface layer of the base steel sheet is calculated by

-   -   dividing OH (g) by ([thickness of the base steel sheet before         polishing (mm)]/[thickness of the base steel sheet after         polishing (mm)]) to calculate the amount of oxygen in solid         solution state contained in the base steel sheet,     -   then subtracting the amount of oxygen in solid solution state         contained in the base steel sheet from the total amount of         oxygen OI (g) contained in the base steel sheet,     -   and then dividing the value by [surface area of the base steel         sheet (per surface) (m²)] and then by 2.

The thickness of the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present disclosure is preferably 0.2 mm or more. The thickness is preferably 3.2 mm or less.

Next, the hot-dip galvanized layer of the hot-dip galvanized steel sheet according to one embodiment of the present disclosure will be described.

Fe Content in Hot-Dip Galvanized Layer: 0.40 Mass % or More

It is preferable to contain a large amount of Fe in the hot-dip galvanized layer to improve the coating adhesion. Therefore, the Fe content in the hot-dip galvanized layer is set to 0.40 mass % or more. The Fe content in the hot-dip galvanized layer is preferably 0.50 mass % or more. On the other hand, an excess of Fe in the hot-dip galvanized layer results in the formation of a hard Fe—Zn alloy phase in the hot-dip galvanized layer. As a result, the coating itself is likely to be broken, resulting in deterioration of coating adhesion. Therefore, the Fe content in the hot-dip galvanized layer is preferably 8.0 mass % or less. The Fe content in the hot-dip galvanized layer is more preferably 7.5 mass % or less and even more preferably 7.0 mass % or less.

Coating Weight in Hot-Dip Galvanized Layer: 20 g/m² or More Per Surface

A large coating weight is desirable to improve the corrosion resistance. Therefore, the coating weight is preferably 20 g/m² or more per surface (note that all the coating weight described below is the amount for one surface). The coating weight is more preferably 25 g/m² or more and even more preferably 30 g/m² or more. The upper limit of the coating weight is not particularly limited. However, if the coating weight exceeds 120 g/m², the above effect is saturated. Therefore, the coating weight is preferably 120 g/m² or less.

The Fe content and the coating weight in the hot-dip galvanized layer are measured as follows.

After degreasing the surface of the hot-dip galvanized steel sheet as a test piece, the mass of the test piece is weighed for the first time. Next, two or three drops of inhibitor, which is a corrosion inhibitor for Fe, are added to 30 cc of 1:3 HCl solution (HCl solution with a concentration of 25 vol. %), and then the test piece is immersed in the solution to dissolve the hot-dip galvanized layer of the test piece. After dissolving the hot-dip galvanized layer (when there is no more H₂ gas formed on the surface of the test piece), the solution is collected. After the test piece is collected and dried, the mass of the test piece is weighed for the second time.

The coating weight is calculated by the following formula.

[Coating weight(g/m²)]([mass of the test piece weighed for the first time(g)]−[mass of the test piece weighed for the second time(g)])÷[coated area of the test piece(area covered by the hot-dip galvanized layer in the test piece before dissolving the hot-dip galvanized layer)(m²)]

The masses of Fe, Zn, and Al dissolved in the collected solution (hereinafter referred to as dissolved amount of Fe, dissolved amount of Zn, and dissolved amount of Al) are measured with the inductively coupled plasma (ICP) method, and the Fe content in the hot-dip galvanized layer is determined by the following formula.

[Fe content in the hot-dip galvanized layer(mass %)[dissolved amount of Fe(g)]/([dissolved amount of Fe(g)]+[dissolved amount of Zn(g)]+[dissolved amount of Al(g)])×100

The hot-dip galvanized layer is mainly composed of Zn and is basically composed of Zn and the aforementioned Fe. Depending on the composition of the coating bath, the hot-dip galvanized layer may contain 0.30 mass % or less, specifically 0.15 mass % to 0.30 mass %, of Al. The balance other than Zn, Fe and Al is inevitable impurities. The hot-dip galvanized layer may be provided on only one side or on both sides of the base steel sheet.

Next, the mechanical properties of the hot-dip galvanized steel sheet according to one embodiment of the present disclosure will be described.

The hot-dip galvanized steel sheet according to one embodiment of the present disclosure has a tensile strength (TS) of 750 MPa or more. The tensile strength (TS) is preferably 780 MPa or more. Although the upper limit of the tensile strength is not particularly limited, a tensile strength of less than 980 MPa is preferred considering the balance with other properties.

Further, from the viewpoint of workability,

-   -   TS×El is 18000 MPa·% or more,     -   TS×λ is 40000 MPa·%, and     -   yield ratio YR (=YS/TS) is 0.55 or more.     -   TS×El is preferably 19000 MPa·% or more and more preferably         20000 MPa·% or more.     -   TS×λ is preferably 45000 MPa·% or more and more preferably 50000         MPa·% or more.     -   YR is preferably 0.60 or more and more preferably 0.65 or more.

As used herein, the tensile strength (TS), the yield stress (YS), and the elongation (El) are measured as follows.

A JIS No. 5 test piece with a gauge length of 50 mm and a gauge width of 25 mm is collected from the center of the width of the hot-dip galvanized steel sheet, with the rolling direction being the longitudinal direction. Next, the collected JIS No. 5 test piece is subjected to a tensile test in accordance with the provisions of JIS Z 2241 (2011) to measure the tensile strength (TS), the yield stress (YS), and the elongation (El). The tensile speed is 10 mm/min.

Further, λ is the maximum hole expansion ratio (%), which is measured as follows.

A 100 mm square test piece is collected from the center of the width of the hot-dip galvanized steel sheet. Next, the collected test piece is subjected to a hole expanding test according to the Japan Iron and Steel Federation standard JFST1001 to measure k. Specifically, after punching a hole with a diameter of 10 mm in the test piece, a 60-degree conical punch is pressed into the hole while the surrounding area is being restrained, and the diameter of the hole at the crack initiation limit is measured. The maximum hole expansion ratio λ (%) is determined by the following formula.

Maximum hole expansion ratioλ(%)={(D _(f) −D ₀)/D ₀}×100

-   -   where D_(f) is the diameter of the hole at the crack initiation         limit (mm), and D₀ is the initial (before the punch is pressed         in) diameter of the hole (mm).

“Excellent coating quality” means that there is no peeling of the hot-dip galvanized layer in a ball impact test under the following conditions, and that there is no non-coating defect in the hot-dip galvanized layer (preferably, there is no uneven coating appearance) found by appearance observation. The non-coating defect refers to an area of several micrometers to several millimeters in size where the base steel sheet is exposed without the hot-dip galvanized layer.

Conditions of Ball Impact Test

-   -   Ball mass: 2.8 kg, drop height: 1 m         (After dropping the ball under the above conditions and causing         the ball to impact the hot-dip galvanized steel sheet, the area         that has been impacted by the ball is peeled by tape (tape with         an adhesive strength of 8 N per 25 mm width in accordance with         JIS Z 1522 (2009)), and the peeling of the hot-dip galvanized         layer is determined visually.)

Next, a method of manufacturing a hot-dip galvanized steel sheet according to one embodiment of the present disclosure will be described.

The method of manufacturing a hot-dip galvanized steel sheet according to one embodiment of the present disclosure comprises

-   -   a hot rolling process where a steel slab having the chemical         composition described above is subjected to hot rolling to         obtain a hot-rolled steel sheet,     -   a cold rolling process where the hot-rolled steel sheet is         subjected to cold rolling to obtain a cold-rolled steel sheet,     -   an annealing process where the cold-rolled steel sheet is heated         to an annealing temperature, held at the annealing temperature,         and then cooled, and     -   then a coating treatment process where the cold-rolled steel         sheet is subjected to hot-dip galvanizing treatment.

In the following description, “temperature” is the surface temperature of the steel sheet or slab unless otherwise specified. The surface temperature of the steel sheet or slab is measured, for example, using a radiation thermometer.

Hot Rolling Process

In this process, a steel material (steel slab) having the chemical composition described above is subjected to hot rolling to obtain a hot-rolled steel sheet.

The steel material used is preferably obtained by continuous casting to prevent macro-segregation of components. The steel material can also be obtained by ingot casting or thin slab casting.

The following describes the optimum manufacturing conditions of the hot rolling process.

Slab Heating Temperature: 1200° C. or Higher

If the heating temperature of the slab is lower than 1200° C., precipitates such as AlN are not sufficiently dissolved. As a result, precipitates such as AlN may be coarsened during the hot rolling, which deteriorates the hole expansion formability. Therefore, the heating temperature of the slab is preferably 1200° C. or higher. The heating temperature of the slab is more preferably 1230° C. or higher and even more preferably 1250° C. or higher. The upper limit of the heating temperature of the slab is not particularly limited, but 1400° C. or lower is preferred. The heating temperature of the slab is more preferably 1350° C. or lower.

Rolling Finish Temperature: 840° C. or Higher and 900° C. or Lower

If the rolling finish temperature is lower than 840° C., inclusions and coarse carbides may be formed, which deteriorates the hole expansion formability. The quality of the interior of the base steel sheet may also be deteriorated. Therefore, the rolling finish temperature is preferably 840° C. or higher. The rolling finish temperature is more preferably 860° C. or higher. On the other hand, if the holding time at high temperatures is increased, coarse inclusions may be formed, which deteriorates the hole expansion formability. Therefore, the rolling finish temperature is preferably 900° C. or lower. The rolling finish temperature is more preferably 880° C. or lower.

Coiling Temperature: 450° C. or Higher and 650° C. or Lower

The steel material is subjected to hot rolling as described above to obtain a hot-rolled steel sheet, and then the hot-rolled steel sheet is coiled. When the coiling temperature is higher than 650° C., the surface of the steel substrate may be decarburized. This may cause a difference in microstructure between the interior and the surface of the base steel sheet, resulting in uneven alloy concentration. Further, coarse carbides and nitrides may be formed, which deteriorates the hole expansion formability. Therefore, the coiling temperature is preferably 650° C. or lower. The coiling temperature is more preferably 630° C. or lower. On the other hand, the coiling temperature is preferably 450° C. or higher to prevent deterioration of cold rolling manufacturability. The coiling temperature is more preferably 470° C. or higher.

The hot-rolled steel sheet may be subjected to pickling after coiling. The conditions of the pickling are not particularly limited, and conventional methods may be followed. Further, the hot-rolled steel sheet may be subjected to heat treatment after coiling to soften the microstructure.

Cold Rolling Process

In this process, the hot-rolled steel sheet obtained in the hot rolling process is subjected to cold rolling to obtain a cold-rolled steel sheet. There is no limit on the cold rolling ratio if the sheet thickness is controlled within a desired range. However, if the cold rolling ratio is too small, it is difficult to cause recrystallization in the subsequent annealing process. That is, non-recrystallized ferrite may be formed, which deteriorates the elongation. Therefore, the cold rolling ratio is preferably 20% or more. The cold rolling ratio is more preferably 30% or more. On the other hand, if the cold rolling ratio is too high, it is also difficult to cause recrystallization in the subsequent annealing process due to excessive strain. That is, non-recrystallized ferrite may be formed, which deteriorates the elongation. Therefore, the cold rolling ratio is preferably 90% or less. The cold rolling ratio is more preferably 80% or less.

Annealing Process

In this process, the cold-rolled steel sheet obtained in the cold rolling process is heated to an annealing temperature, held at the annealing temperature, and then cooled.

Further, from the viewpoint of creating a complex structure as described above, forming oxides of Si and Mn in the surface layer of the base steel sheet by causing internal oxidation in the surface layer of the base steel sheet, and containing an appropriate amount of Fe in the hot-dip galvanized layer, it is important in this process to set

-   -   the average heating rate in a temperature range from 500° C. to         the annealing temperature during the heating (hereinafter also         referred to as “average heating rate”) to 1° C./s or higher and         7° C./s or lower,     -   the annealing temperature to (A_(C1) point+50° C.) or higher and         (A_(C3) point+20° C.) or lower,     -   the holding time during the holding (hereafter also referred to         as “annealing time”) to 1 second or longer and 40 seconds or         shorter,     -   during the holding, the dew point of the atmosphere to −20° C.         or higher and 5° C. or lower, and the hydrogen concentration of         the atmosphere to 3 mass % or more and 20 mass % or less,     -   the average cooling rate in a temperature range from the         annealing temperature to a primary cooling stop temperature         during the cooling (hereinafter also referred to as “primary         cooling rate”) to 10° C./s or higher,     -   the primary cooling stop temperature to 450° C. or higher and         600° C. or lower,     -   the secondary cooling time (the time from reaching the primary         cooling stop temperature to reaching a secondary cooling stop         temperature (if the primary cooling stop temperature is equal to         the secondary cooling stop temperature, it refers to the staying         time at that temperature after reaching the primary cooling stop         temperature)) to 20 seconds or longer and 100 seconds or         shorter, and     -   the secondary cooling stop temperature to 400° C. or higher and         500° C. or lower.

Average heating rate: 1° C./s or higher and 7° C./s or lower

The average heating rate is preferably a low rate so that ferrite is recrystallized and the desired area ratio of ferrite is ensured. Therefore, the average heating rate is set to 7° C./s or lower. The average heating rate is preferably 6° C./s or lower and more preferably 5° C./s or lower. On the other hand, as the average heating rate decreases, Mn, which diffuses at a low rate, also concentrates into austenite and stabilizes the austenite. As a result, it is difficult to cause bainite transformation, and the desired complex structure cannot be obtained. Therefore, the average heating rate is set to 1° C./s or higher. The average heating rate is preferably 2° C./s or higher and more preferably 3° C./s or higher.

Annealing temperature: (A_(C1) point+50° C.) or higher and (A_(C3) point+20° C.) or lower

If the annealing temperature is lower than (A_(C1) point+50° C.), coarse Fe-based precipitates are formed, which deteriorates the strength and the hole expansion formability. Therefore, the annealing temperature is set to (A_(C1) point+50° C.) or higher. The annealing temperature is preferably (A_(C1) point+60° C.) or higher. On the other hand, if the annealing temperature exceeds (A_(C3) point+20° C.), the area ratio of ferrite decreases, and the elongation deteriorates. Therefore, the annealing temperature is set to (A_(C3) point+20° C.) or lower. The annealing temperature is preferably (A_(C3) point+10° C.) or lower.

As used herein, the A_(C1) point and the A_(C3) point are calculated by the following formulas, respectively. Note that in the following formulas, (% element symbol) refers to the content (mass %) of each element in the chemical composition of the base steel sheet. If the element is not contained (including cases where it is inevitably contained), it is calculated as 0.

A_(C1)=723+22(% Si)−18(% Mn)+17(% Cr)+4.5(% Mo)+16(% V)

A_(C3)=910−203√(% C)+45(% Si)−30(% Mn)−20(% Cu)−15(% Ni)+11(% Cr)+32(% Mo)+104(% V)+400(% Ti)+460(% Al)

The annealing temperature may be constant during the holding. The annealing temperature may not be constant during the holding, if it is within the above temperature range and the temperature fluctuation range is within ±10° C. of the set temperature.

Annealing time: 1 second or longer and 40 seconds or shorter

The annealing time is an important condition to transform austenite to bainite. From the viewpoint of avoiding concentration of Mn in austenite, i.e., avoiding excessive stabilization of austenite and obtaining an appropriate amount of bainite, the annealing time is preferably short. Therefore, the annealing time is set to 40 seconds or shorter. The annealing time is preferably 30 seconds or shorter and more preferably 25 seconds or shorter. On the other hand, if the annealing time is shorter than 1 second, recrystallization of ferrite is not promoted, resulting in deteriorated hole expansion formability. Therefore, the annealing time is set to 1 second or longer. The annealing time is preferably 5 seconds or longer. The annealing time is the holding time at the annealing temperature.

Dew Point of Holding Atmosphere: −20° C. or Higher and 5° C. or Lower

As described above, it is necessary to ensure a certain amount of oxygen in the holding atmosphere to cause internal oxidation in the surface layer of the base steel sheet and to form appropriate amounts of Si and Mn oxides in the surface layer of the base steel sheet. Further, it is necessary to raise the dew point to some extent from the viewpoint of ensuring an appropriate amount of Fe in the hot-dip galvanized layer. Therefore, the dew point of the holding atmosphere is set to −20° C. or higher. The dew point of the holding atmosphere is preferably −18° C. or higher and more preferably −15° C. or higher. On the other hand, if the dew point is too high, excessive internal oxidation is caused in the surface layer of the base steel sheet, which deteriorates the elongation and the hole expansion formability. If the dew point is too high, iron diffusion is excessively promoted during the coating treatment, resulting in excessive diffusion of iron in the coated layer. Therefore, the dew point of the holding atmosphere is set to 5° C. or lower. The dew point of the holding atmosphere is preferably 0° C. or lower.

Hydrogen Concentration in Holding Atmosphere: 3 Mass % or More and 20 Mass % or Less.

To promote internal oxidation in the surface layer of the base steel sheet and to ensure the coating weight of the hot-dip galvanized layer, the oxides formed on the surface of the base steel sheet (and formed during the holding of the annealing process) need to be reduced. Therefore, the hydrogen concentration in the holding atmosphere is set to 3 mass % or more. The hydrogen concentration in the holding atmosphere is preferably 5 mass % or more. On the other hand, if the hydrogen concentration in the holding atmosphere is too high, hydrogen penetrates into the steel, and the elongation and the hole expansion formability are deteriorated. Therefore, the hydrogen concentration in the holding atmosphere is set to 20 mass % or less. The hydrogen concentration in the holding atmosphere is preferably 17 mass % or less.

Primary Cooling Rate: 10° C./s or Higher

During the cooling process in a temperature range from the annealing temperature to the primary cooling stop temperature, it is necessary to properly control the cooling rate to form bainite. That is, if the primary cooling rate is low, pearlite is formed in addition to ferrite, and an appropriate amount of bainite cannot be obtained. Therefore, the primary cooling rate is set to 10° C./s or higher. The primary cooling rate is preferably 12° C./s or higher and more preferably 15° C./s or higher. The upper limit of the primary cooling rate is not limited, because a high primary cooling rate is preferred to suppress pearlite transformation. For example, there is no problem if the primary cooling rate reaches 2000° C./s or higher by water cooling or like.

Primary Cooling Stop Temperature: 450° C. or Higher and 600° C. or Lower

The primary cooling stop temperature is set to 450° C. or higher and 600° C. or lower to suppress pearlite transformation during the primary cooling and to ensure the specified amount of bainite during the secondary cooling. That is, if the primary cooling stop temperature exceeds 600° C., pearlite transformation is accelerated during the secondary cooling. Therefore, the primary cooling stop temperature is set to 600° C. or lower. The primary cooling stop temperature is preferably 580° C. or lower and more preferably 560° C. or lower. On the other hand, if the primary cooling stop temperature is lower than 450° C., bainite transformation is suppressed during the secondary cooling, rendering it difficult to ensure the specified fraction of bainite. Therefore, the primary cooling stop temperature is set to 450° C. or higher. The primary cooling stop temperature is preferably 460° C. or higher and more preferably 470° C. or higher.

Secondary Cooling Time: 20 Seconds or Longer and 100 Seconds or Shorter

In the secondary cooling process from the primary cooling stop temperature to the secondary cooling stop temperature following the primary cooling process, it is necessary properly control the secondary cooling time to form bainite. That is, a long secondary cooling time promotes bainite transformation. Therefore, the secondary cooling time is set to 20 seconds or longer. The secondary cooling time is preferably 25 seconds or longer and more preferably 30 seconds or longer. On the other hand, if the secondary cooling time is too long, bainite is excessively formed, and the area ratio of martensite necessary for ensuring strength cannot be obtained. Therefore, the secondary cooling time is set to 100 seconds or shorter. The secondary cooling time is preferably 90 seconds or shorter and more preferably 80 seconds or shorter.

Secondary Cooling Stop Temperature: 400° C. or Higher and 500° C. or Lower

The secondary cooling stop temperature is set to 400° C. or higher and 500° C. or lower from the viewpoint of ensuring the specified fraction of bainite and controlling the temperature of the cold-rolled steel sheet when it enters the coating bath in the coating treatment process, which will be described later, within the specified range. That is, if the secondary cooling stop temperature exceeds 500° C., bainite transformation is accelerated during the secondary cooling, and the fraction of bainite becomes too high. Therefore, the secondary cooling stop temperature is set to 500° C. or lower. The secondary cooling stop temperature is preferably 495° C. or lower and more preferably 490° C. or lower. On the other hand, if the secondary cooling stop temperature is lower than 400° C., it is difficult to control the temperature of the cold-rolled steel sheet when it enters the coating bath to a temperature at least 10° C. higher than the coating bath temperature even if heat treatment is applied immediately before the coating treatment, especially in a case of using a continuous annealing hot-dip galvanizing line (CGL). Therefore, the secondary cooling stop temperature is set to 400° C. or higher. The secondary cooling stop temperature is preferably 420° C. or higher and more preferably 440° C. or higher.

Coating Treatment Process

In this process, the cold-rolled steel sheet is subjected to hot-dip galvanizing treatment after the annealing treatment.

Further, in this process, it is important that the temperature of the cold-rolled steel sheet when it enters the coating bath be at least 10° C. higher than the coating bath temperature.

Temperature of the cold-rolled steel sheet when it enters the coating bath: coating bath temperature+10° C. or higher

To ensure an appropriate amount of Fe in the hot-dip galvanized layer, it is necessary to control the temperature of the cold-rolled steel sheet when it enters the coating bath higher than the coating bath temperature, especially to a temperature at least 10° C. higher than the coating bath temperature. The temperature of the cold-rolled steel sheet when it enters the coating bath is preferably at least 15° C. higher than the coating bath temperature and more preferably at least 20° C. higher than the coating bath temperature. The upper limit of the temperature of the cold-rolled steel sheet when it enters the coating bath is not particularly limited, but it is preferably 500° C. or lower.

The coating bath is basically composed of Zn, and it may contain 0.15 mass % to 0.30 mass % of Al. The balance other than Zn and Al is inevitable impurities.

The Coating Bath Temperature is Preferably 440° C. to 500° C.

In addition, the annealing process and the coating treatment process may be performed on a continuous annealing line (CAL) or on a continuous annealing hot-dip galvanizing line (CGL). Each process may be performed by batch processing.

The conditions of each process other than the above are not limited, and conventional methods may be followed. After the annealing process, temper rolling may be performed for shape adjustment.

According to the above manufacturing method, it is possible to obtain a hot-dip galvanized steel sheet that has both high strength and good workability as well as excellent coating quality, and this hot-dip galvanized steel sheet can be suitably used for automotive members.

Examples

Steel materials having the chemical compositions listed in Table 1 (with the balance being Fe and inevitable impurities) were melted in a vacuum melting furnace and then subjected to blooming to obtain bloomed materials with a thickness of 27 mm. The obtained bloomed materials were subjected to hot rolling under the conditions listed in Table 2 to obtain hot-rolled steel sheets with a thickness of 4.0 mm. Next, the obtained hot-rolled steel sheets were ground to a thickness of 3.0 mm, and then they were subjected to cold rolling under the conditions listed in Table 2 to obtain cold-rolled steel sheets with a thickness 0.9 mm to 1.8 mm. Next, the obtained cold-rolled steel sheets were subjected to annealing and coating treatment under the conditions listed in Table 2 to obtain hot-dip galvanized steel sheets with hot-dip galvanized layers on both sides. Blank cells in Table 1 indicate that the element is not intentionally added (it is not necessarily 0 mass %, and it may be inevitably contained).

Next, each obtained hot-dip galvanized steel sheet was used to identify the microstructure in the base steel sheet, measure the amount of oxygen in oxide form in the surface layer of the base steel sheet, and measure the coating weight and the Fe content per surface in the hot-dip galvanized layer, according to the procedure described above.

The results are listed in Table 3.

For the identification of the microstructure in the base steel sheet (point counting method), 16×15 grids were evenly spaced over an area to be observed by a SEM (an area of 82 μm×57 μm). The number of grid points in each phase was counted, and a ratio of the number of grid points occupied by each phase to the total number of grid points was taken as the area ratio of each phase. Note that the area ratio of each phase was the average value of the area ratios of each phase obtained from three separate SEM images.

Further, the mechanical properties of each of the obtained hot-dip galvanized steel sheets were measured according to the procedure described above. The results are listed in Table 4.

The Desired Tensile Strength (TS) is 750 MPa or More.

From the viewpoint of workability, the desired TS×El is 18000 MPa·% 20 or more, TS×λ is 40000 MPa·% or more, and yield ratio YR (=YS/TS) is 0.55 or more.

Furthermore, the coating quality (coating adhesion and coating appearance) of each of the obtained hot-dip galvanized steel sheets was examined according to the procedure described above and evaluated according to the following criteria. The evaluation results are listed in Table 4.

Coating Adhesion

-   -   ◯ (Passed and excellent): the hot-dip galvanized layer did not         peel off in the ball impact test as described above.     -   x (Failed): the hot-dip galvanized layer peeled off in the ball         impact test as described above.

Coating Appearance

-   -   ⊚ (Passed and extremely excellent): there was no non-coating         defect or uneven coating appearance in the hot-dip galvanized         layer.     -   ∘ (Passed and excellent): there was uneven coating appearance in         the hot-dip galvanized layer, but there was no non-coating         defect in the hot-dip galvanized layer.     -   x (Failed): there was a non-coating defect in the hot-dip         galvanized layer.

TABLE 1 Steel Chemical composition (mass %) A_(c1) point A_(c3) point sample ID C Si Mn P S Al N Nb Ti B Cr Mo V Others (° C.) (° C.) A 0.13 0.9 2.1 0.013 0.007 0.02 0.004 705 824 B 0.10 0.8 2.3 0.020 0.013 0.05 0.009 699 836 C 0.16 0.8 2.4 0.010 0.007 0.10 0.005 697 839 D 0.15 0.4 2.3 0.018 0.008 0.02 0.010 690 790 E 0.13 1.0 2.4 0.014 0.012 0.03 0.006 702 824 F 0.14 0.5 2.0 0.018 0.004 0.04 0.009 698 815 G 0.15 0.8 2.6 0.017 0.014 0.06 0.008 694 817 H 0.12 0.7 2.1 0.012 0.009 0.08 0.008 0.0015 701 845 I 0.11 0.9 2.3 0.010 0.004 0.02 0.006 0.012 0.0010 702 828 J 0.14 0.5 2.2 0.005 0.005 0.02 0.009 0.02 694 800 K 0.11 0.5 2.4 0.017 0.005 0.07 0.008 0.025 691 825 L 0.12 0.9 2.4 0.019 0.006 0.03 0.008 0.02 0.05 700 823 M 0.12 0.9 2.4 0.013 0.014 0.03 0.002 0.015 0.02 700 830 N 0.18 0.6 2.4 0.009 0.013 0.07 0.009 693 811 O 0.08 0.6 2.2 0.008 0.006 0.02 0.009 697 823 P 0.14 1.2 2.3 0.014 0.009 0.04 0.003 708 837 Q 0.15 0.2 2.5 0.010 0.010 0.09 0.009 682 807 R 0.11 0.8 2.8 0.012 0.014 0.06 0.008 690 822 S 0.11 0.7 1.8 0.019 0.007 0.02 0.004 706 829 T 0.14 0.7 2.2 0.013 0.003 0.25 0.009 699 915 U 0.12 0.5 2.4 0.015 0.013 0.02 0.120 691 799 V 0.13 0.9 2.1 0.013 0.007 0.02 0.004 Ta: 0.01 705 824 W 0.13 0.9 2.1 0.013 0.007 0.02 0.004 0.02 W: 0.01 705 824 X 0.13 0.9 2.1 0.013 0.007 0.02 0.004 Ni: 0.01 705 823 Y 0.13 0.9 2.1 0.013 0.007 0.02 0.004 0.015 0.0010 Cu: 0.05 705 829 Z 0.13 0.9 2.1 0.013 0.007 0.02 0.004 0.0010 Sb: 0.01 705 824 AA 0.11 0.8 2.3 0.013 0.007 0.02 0.004 Sn: 0.01 699 819 AB 0.11 0.8 2.3 0.013 0.007 0.02 0.004 0.10 Ca: 0.003 701 820 AC 0.11 0.8 2.3 0.013 0.007 0.02 0.004 Mg: 0.002 699 819 AD 0.11 0.8 2.3 0.013 0.007 0.02 0.004 0.02 Zr: 0.05 700 821

TABLE 2 Hot rolling Annealing Slab Rolling Cold rolling Average Steel heating finish Coiling Cold rolling heating Annealing Annealing Dew Hydrogen sample temperature temperature temperature ratio rate temperature time point concentration No. ID ° C. ° C. ° C. % ° C./s ° C. second ° C. mass % 1 A 1250 880 550 50 3 820 5 −12 5 2 1250 880 550 50 5 820 14 −22 12 3 1250 880 550 50 3 820 35 −18 13 4 1250 880 550 50 5 820 30 −7 6 5 B 1250 880 550 50 5 840 15 −13 3 6 1230 880 550 50 3 840 14 −11 8 7 1220 880 550 50 3 840 16 −6 9 8 1200 880 550 50 4 840 11 −8 14 9 C 1250 880 550 50 3 840 0.3 −8 9 10 1250 840 550 50 5 840 11 −12 13 11 1250 900 550 50 4 840 46 −11 9 12 1250 840 550 50 5 840 38 −11 5 13 D 1250 880 550 50 4 800 19 −11 15 14 1250 880 450 50 4 800 18 −10 13 15 1250 880 650 50 4 800 13 −13 11 16 1250 880 480 50 4 800 20 −9 6 17 E 1250 880 550 40 5 820 11 −15 15 18 1250 880 550 50 5 820 20 −14 2 19 1250 880 550 60 5 820 11 −11 22 20 1250 880 550 70 5 820 19 −10 6 21 F 1250 880 550 50 0.4 820 12 −9 9 22 1250 880 550 50 5 820 17 −15 14 23 1250 880 550 50 8 820 14 −5 10 24 1250 880 550 50 7 820 18 −13 7 25 G 1250 880 550 50 3 820 12 −12 11 26 1250 880 550 50 5 730 11 −13 15 27 1250 880 550 50 4 780 12 −5 13 28 1250 880 550 50 5 860 19 −6 5 29 H 1250 880 550 50 4 860 12 −11 12 30 1250 880 550 50 3 860 0.4 −9 8 31 1250 880 550 50 5 860 18 −5 13 32 1250 880 550 50 5 860 4 −15 11 33 I 1250 880 550 50 5 840 13 8 13 34 1250 880 550 50 4 840 16 −3 13 35 1250 880 550 50 4 840 10 2 12 36 1250 880 550 50 5 840 19 −11 11 37 J 1250 880 550 50 3 820 13 −7 12 38 1250 880 550 50 5 820 13 −8 14 39 1250 880 550 50 5 820 19 −9 9 40 1250 880 550 50 3 820 18 −6 6 41 K 1250 880 500 50 2 840 18 −12 9 42 1250 880 600 50 3 840 16 −15 14 43 1250 880 650 50 6 840 15 −6 11 44 1250 880 550 50 4 840 20 −9 5 45 L 1250 880 550 50 4 820 20 −14 12 46 1250 880 550 50 5 820 20 −15 6 47 1250 880 550 50 5 820 25 −15 14 48 1250 880 550 50 3 820 35 −15 11 49 M 1250 880 550 50 5 820 19 −12 8 50 1250 880 550 50 4 820 12 −7 5 51 1250 880 550 50 3 820 18 −9 6 52 1250 880 550 50 3 820 14 −9 10 53 N 1250 880 550 50 5 830 10 −11 14 54 O 1250 880 550 50 4 840 19 −9 8 55 P 1250 880 550 50 5 850 14 −8 13 56 Q 1250 880 550 50 4 820 19 −5 14 57 R 1250 880 550 50 4 820 13 −12 10 58 S 1250 880 550 50 5 830 14 −10 8 59 T 1250 880 550 50 4 900 10 −12 9 60 U 1250 880 550 50 3 800 20 −15 6 61 A 1250 880 550 50 5 820 4 −10 5 62 A 1250 880 550 50 5 830 10 −10 5 63 A 1250 880 550 50 5 820 2 −15 5 64 V 1250 880 550 50 3 820 5 −12 5 65 W 1250 880 550 50 5 840 15 −13 3 66 X 1250 880 550 50 3 820 5 −12 5 67 Y 1250 880 550 50 5 840 15 −13 3 68 Z 1250 840 550 50 5 840 11 −12 13 69 AA 1250 880 550 50 3 820 5 −12 5 70 AB 1250 880 550 50 5 800 15 −13 3 71 AC 1250 840 550 50 5 800 11 −12 13 72 AD 1250 880 550 50 3 820 5 −12 5 Coating treatment Annealing Sheet Primary Secondary temperature Primary cooling Secondary cooling when Coating cooling stop cooling stop entering bath rate temperature time temperature coating temperature No. ° C./s ° C. second ° C. ° C. ° C. Remarks 1 28 530 60 470 480 460 Example 2 27 520 40 490 480 460 Comparative Example 3 28 430 50 410 480 460 Comparative Example 4 30 630 70 450 480 460 Comparative Example 5 27 470 40 470 490 460 Example 6 26 560 50 490 470 460 Example 7 30 480 70 480 460 460 Comparative Example 8 29 490 60 490 490 460 Example 9 25 550 60 460 490 460 Comparative Example 10 23 480 70 480 490 460 Example 11 27 530 40 460 490 460 Comparative Example 12 28 520 40 460 490 460 Example 13 23 490 70 490 480 460 Example 14 39 550 40 480 480 460 Example 15 1400 500 70 490 480 460 Example 16 125 540 40 450 480 460 Example 17 20 530 70 460 480 460 Example 18 22 510 50 480 480 460 Comparative Example 19 29 490 50 470 480 460 Comparative Example 20 20 500 40 450 480 460 Example 21 22 550 50 450 490 460 Comparative Example 22 21 510 70 460 490 460 Example 23 21 490 60 490 490 460 Comparative Example 24 24 550 60 470 490 460 Example 25 29 510 70 470 490 460 Example 26 27 500 40 450 490 460 Comparative Example 27 25 530 40 450 490 460 Example 28 22 530 70 460 490 460 Comparative Example 29 22 500 50 460 490 460 Example 30 30 520 60 470 490 460 Comparative Example 31 22 480 60 460 490 460 Example 32 26 480 60 450 480 460 Example 33 25 500 40 470 480 460 Comparative Example 34 26 490 50 460 490 460 Example 35 25 490 60 370 460 460 Comparative Example 36 24 540 70 530 490 460 Comparative Example 37 28 510 40 440 490 460 Example 38 12 540 40 500 490 460 Example 39 6 490 60 480 490 460 Comparative Example 40 17 490 60 490 490 460 Example 41 29 490 30 450 490 460 Example 42 24 520 10 470 490 460 Comparative Example 43 22 480 40 470 490 460 Example 44 23 530 20 490 490 460 Example 45 25 520 60 490 460 460 Comparative Example 46 22 550 40 490 490 460 Example 47 25 500 60 490 470 460 Example 48 22 510 40 450 500 460 Example 49 26 550 40 460 490 460 Example 50 20 510 120 480 490 460 Comparative Example 51 22 520 100 450 490 460 Example 52 20 530 80 460 490 460 Example 53 20 520 60 480 490 460 Comparative Example 54 23 530 40 460 490 460 Comparative Example 55 20 540 40 450 490 460 Comparative Example 56 23 530 50 450 490 460 Comparative Example 57 23 550 50 460 490 460 Comparative Example 58 20 490 70 490 490 460 Comparative Example 59 26 530 40 450 490 460 Comparative Example 60 20 520 70 460 490 460 Comparative Example 61 40 490 100 460 480 460 Example 62 15 540 70 480 490 460 Example 63 10 550 80 480 490 460 Example 64 28 530 60 470 480 460 Example 65 27 470 40 470 490 460 Example 66 28 530 60 470 480 460 Example 67 27 470 40 470 490 460 Example 68 23 480 70 480 490 460 Example 69 28 530 60 470 480 460 Example 70 27 470 40 470 490 460 Example 71 23 480 70 480 490 460 Example 72 28 530 60 470 480 460 Example

TABLE 3 Amount of oxygen Hot-dip Steel microstructure of base steel sheet in oxide form galvanized layer Steel Other metallic phases in surface layer Fe Coating sample α M B P Non-α Retained of base steel sheet content weight No. ID % % % Total % % % γ % g/m² mass % g/m² Remarks 1 A 54 14 32 0 0 0 0 0.25 0.91 31 Example 2 54 14 32 0 0 0 0 0.03 0.39 45 Comparative Example 3 55 35 7 3 0 3 0 0.06 0.52 45 Comparative Example 4 56 14 24 6 6 0 0 0.24 1.00 30 Comparative Example 5 B 55 26 18 1 1 0 0 0.21 0.93 22 Example 6 55 14 28 3 3 0 0 0.18 0.45 41 Example 7 53 14 33 0 0 0 0 0.16 0.28 44 Comparative Example 8 56 16 28 0 0 0 0 0.21 0.96 47 Example 9 C 37 27 25 11 0 11 0 0.26 0.99 44 Comparative Example 10 52 25 23 0 0 0 0 0.30 0.97 43 Example 11 56 35 6 3 1 1 1 0.20 0.97 46 Comparative Example 12 48 24 28 0 0 0 0 0.27 0.92 27 Example 13 D 55 18 27 0 0 0 0 0.23 0.85 44 Example 14 54 21 25 0 0 0 0 0.23 0.89 47 Example 15 56 22 22 0 0 0 0 0.27 0.90 46 Example 16 55 18 27 0 0 0 0 0.27 0.83 34 Example 17 E 57 17 26 0 0 0 0 0.39 0.97 47 Example 18 57 15 28 0 0 0 0 0.04 0.91 17 Comparative Example 19 55 13 32 0 0 0 0 0.57 0.96 47 Comparative Example 20 54 16 30 0 0 0 0 0.39 0.90 27 Example 21 F 73 20 7 0 0 0 0 0.27 0.99 46 Comparative Example 22 53 15 32 0 0 0 0 0.21 0.98 45 Example 23 45 13 30 12 0 12 0 0.23 2.86 44 Comparative Example 24 51 16 30 3 0 3 0 0.26 0.92 30 Example 25 G 54 20 26 0 0 0 0 0.31 0.88 46 Example 26 96 4 0 0 0 0 0 0.37 0.88 45 Comparative Example 27 63 11 26 0 0 0 0 0.40 2.73 43 Example 28 24 40 36 0 0 0 0 0.34 1.74 26 Comparative Example 29 H 54 15 31 0 0 0 0 0.25 1.00 46 Example 30 46 17 27 10 0 10 0 0.25 0.84 41 Comparative Example 31 57 16 27 0 0 0 0 0.25 2.71 47 Example 32 57 13 30 0 0 0 0 0.22 0.85 45 Example 33 I 56 14 30 0 0 0 0 0.68 6.40 47 Comparative Example 34 55 14 31 0 0 0 0 0.45 4.81 45 Example 35 54 16 30 0 0 0 0 0.46 0.35 46 Comparative Example 36 34 2 64 0 0 0 0 0.30 1.01 44 Comparative Example 37 J 54 14 32 0 0 0 0 0.21 1.03 47 Example 38 57 13 27 3 0 2 1 0.26 1.01 46 Example 39 58 13 15 14 14 0 0 0.29 0.93 44 Comparative Example 40 54 16 30 0 0 0 0 0.29 1.78 28 Example 41 K 55 19 26 0 0 0 0 0.22 0.88 45 Example 42 56 40 4 0 0 0 0 0.25 0.93 47 Comparative Example 43 53 15 32 0 0 0 0 0.23 1.83 44 Example 44 58 24 18 0 0 0 0 0.30 0.99 25 Example 45 L 53 15 32 0 0 0 0 0.24 0.29 45 Comparative Example 46 53 17 30 0 0 0 0 0.26 0.88 30 Example 47 57 16 27 0 0 0 0 0.08 0.51 45 Example 48 55 18 27 0 0 0 0 0.29 1.01 45 Example 49 M 55 14 31 0 0 0 0 0.26 0.92 42 Example 50 53 2 45 0 0 0 0 0.22 0.81 29 Comparative Example 51 56 5 39 0 0 0 0 0.30 0.81 30 Example 52 57 11 32 0 0 0 0 0.24 0.82 45 Example 53 N 45 49 6 0 0 0 0 0.22 0.90 46 Comparative Example 54 O 57 15 28 0 0 0 0 0.25 0.84 45 Comparative Example 55 P 53 16 31 0 0 0 0 0.60 0.83 44 Comparative Example 56 Q 53 14 33 0 0 0 0 0.21 2.80 47 Comparative Example 57 R 54 15 31 0 0 0 0 0.67 0.91 47 Comparative Example 58 S 57 16 27 0 0 0 0 0.30 0.88 44 Comparative Example 59 T 57 16 27 0 0 0 0 0.25 0.95 45 Comparative Example 60 U 53 16 31 0 0 0 0 0.23 0.98 30 Comparative Example 61 A 42 18 35 5 0 0 5 0.28 0.98 45 Example 62 A 55 14 22 9 0 0 9 0.24 1.01 48 Example 63 A 40 15 31 14 0 0 14 0.27 0.99 39 Example 64 V 52 16 32 0 0 0 0 0.23 0.89 44 Example 65 W 48 15 37 0 0 0 0 0.24 0.91 48 Example 66 X 55 11 34 0 0 0 0 0.22 0.84 49 Example 67 Y 49 16 35 0 0 0 0 0.29 0.86 45 Example 68 Z 51 17 32 0 0 0 0 0.30 0.92 42 Example 69 AA 56 13 31 0 0 0 0 0.28 0.91 44 Example 70 AB 44 16 40 0 0 0 0 0.24 0.98 46 Example 71 AC 56 14 30 0 0 0 0 0.26 0.95 47 Example 72 AD 54 12 34 0 0 0 0 0.25 0.87 45 Example α: area ratio of ferrite, M: area ratio of martensite, B: area ratio of bainite, P: area ratio of pearlite, Non-α: area ratio of non-recrystallized ferrite, retained γ: area ratio of retained austenite

TABLE 4 Steel Mechanical properties Coating quality sample YS TS TS × El TS × λ Coating Coating No. ID MPa MPa YR MPa · % MPa · % adhesion appearance Remarks 1 A 551 820 0.67 21200 50900 ∘ ⊚ Example 2 554 822 0.67 20300 50000 x ⊚ Comparative Example 3 546 1032 0.53 20100 38600 ∘ ⊚ Comparative Example 4 554 823 0.67 17400 38400 ∘ ⊚ Comparative Example 5 B 565 928 0.61 20800 46500 ∘ ∘ Example 6 563 830 0.68 19600 46100 ∘ ⊚ Example 7 561 825 0.68 20700 50900 x ⊚ Comparative Example 8 555 829 0.67 20000 46900 ∘ ⊚ Example 9 C 630 1015 0.62 16500 35900 ∘ ⊚ Comparative Example 10 634 1011 0.63 18600 42900 ∘ ⊚ Example 11 534 1036 0.52 18800 35100 ∘ ⊚ Comparative Example 12 629 1010 0.62 18600 45500 ∘ ∘ Example 13 D 604 938 0.64 18800 45900 ∘ ⊚ Example 14 604 941 0.64 19200 48400 ∘ ⊚ Example 15 598 936 0.64 20400 49400 ∘ ⊚ Example 16 596 939 0.63 20200 49600 ∘ ⊚ Example 17 E 625 938 0.67 18300 46900 ∘ ⊚ Example 18 616 940 0.66 21400 50500 x x Comparative Example 19 621 935 0.66 17400 37300 ∘ ⊚ Comparative Example 20 618 937 0.66 20900 50500 ∘ ∘ Example 21 F 434 808 0.54 22500 36700 ∘ ⊚ Comparative Example 22 550 811 0.68 19700 47500 ∘ ⊚ Example 23 572 807 0.71 16900 39500 ∘ ⊚ Comparative Example 24 561 806 0.70 19000 47300 ∘ ⊚ Example 25 G 690 1056 0.65 21000 50000 ∘ ⊚ Example 26 493 729 0.68 18900 37000 ∘ ⊚ Comparative Example 27 690 1015 0.68 19200 47000 ∘ ⊚ Example 28 687 1077 0.64 17900 50000 ∘ ∘ Comparative Example 29 H 550 802 0.69 20700 50000 ∘ ⊚ Example 30 553 805 0.69 17000 38900 ∘ ⊚ Comparative Example 31 552 800 0.69 20400 48900 ∘ ⊚ Example 32 546 804 0.68 20600 51100 ∘ ⊚ Example 33 I 585 860 0.68 17500 38900 ∘ ⊚ Comparative Example 34 592 862 0.69 18300 45200 ∘ ⊚ Example 35 589 858 0.69 18900 40500 x ⊚ Comparative Example 36 587 859 0.68 17500 51100 ∘ ⊚ Comparative Example 37 J 594 894 0.66 20000 50000 ∘ ⊚ Example 38 592 897 0.66 18500 46400 ∘ ⊚ Example 39 586 892 0.66 17300 38600 ∘ ⊚ Comparative Example 40 595 896 0.66 21300 50700 ∘ ∘ Example 41 K 604 892 0.68 20800 51000 ∘ ⊚ Example 42 482 894 0.54 20900 38400 ∘ ⊚ Comparative Example 43 606 890 0.68 20900 51100 ∘ ⊚ Example 44 557 891 0.63 21100 46300 ∘ ∘ Example 45 L 632 924 0.68 21100 50500 x ⊚ Comparative Example 46 631 926 0.68 20600 51300 ∘ ⊚ Example 47 627 921 0.68 19500 49200 ∘ ⊚ Example 48 604 923 0.65 20500 48900 ∘ ⊚ Example 49 M 619 928 0.67 20500 50500 ∘ ⊚ Example 50 524 731 0.72 20500 51900 ∘ ∘ Comparative Example 51 584 825 0.71 20500 51400 ∘ ⊚ Example 52 615 909 0.68 20900 50500 ∘ ⊚ Example 53 N 567 1065 0.53 16500 37900 ∘ ⊚ Comparative Example 54 O 505 735 0.69 20600 50700 ∘ ⊚ Comparative Example 55 P 615 925 0.66 17600 38100 x x Comparative Example 56 Q 564 1030 0.55 20900 50700 ∘ ⊚ Comparative Example 57 R 686 1050 0.65 17100 38400 x x Comparative Example 58 S 430 650 0.66 21000 50500 ∘ ⊚ Comparative Example 59 T 578 885 0.65 19400 37600 ∘ ⊚ Comparative Example 60 U 598 915 0.65 18700 39300 ∘ ⊚ Comparative Example 61 A 589 920 0.64 23000 41000 ∘ ⊚ Example 62 A 509 799 0.64 19000 46000 ∘ ⊚ Example 63 A 509 788 0.65 18200 41000 ∘ ⊚ Example 64 V 555 850 0.65 19700 45200 ∘ ⊚ Example 65 W 583 832 0.70 19500 45200 ∘ ⊚ Example 66 X 490 821 0.60 19900 44500 ∘ ⊚ Example 67 Y 545 870 0.63 19200 47800 ∘ ⊚ Example 68 Z 519 884 0.59 19400 50000 ∘ ⊚ Example 69 AA 484 841 0.58 19600 42100 ∘ ⊚ Example 70 AB 565 842 0.67 19800 43500 ∘ ⊚ Example 71 AC 529 804 0.66 20000 48100 ∘ ⊚ Example 72 AD 478 804 0.59 19600 43500 ∘ ⊚ Example

As listed in Table 4, all Examples had both high strength and good workability, as well as excellent coating quality.

On the other hand, at least one of strength, workability and coating quality was insufficient in Comparative Examples. 

1. A hot-dip galvanized steel sheet comprising a base steel sheet and a hot-dip galvanized layer on a surface of the base steel sheet, wherein the base steel sheet comprises a chemical composition containing, in mass %, C: 0.09% or more and 0.17% or less, Si: 0.3% or more and 1.1% or less, Mn: 1.9% or more and 2.7% or less, P: 0.10% or less, S: 0.050% or less, Al: 0.01% or more and 0.20% or less, and N: 0.10% or less, with the balance being Fe and inevitable impurities, and a steel microstructure where ferrite has an area ratio of 30% or more and 85% or less, martensite has an area ratio of 5% or more and 30% or less, bainite has an area ratio of 10% or more and 60% or less, and other metallic phases have an area ratio of 15% or less with respect to the entire steel microstructure, oxygen is present as oxides in a surface layer of the base steel sheet in an amount of 0.05 g/m² or more and 0.50 g/m² or less per surface, where the surface layer is an area from a surface of the base steel sheet to a position at a depth of 100 μm, and the hot-dip galvanized layer contains Fe in an amount of 0.40 mass % or more.
 2. The hot-dip galvanized steel sheet according to claim 1, wherein the other metallic phases have an area ratio of 5% or less.
 3. The hot-dip galvanized steel sheet according to claim 1, wherein the hot-dip galvanized layer contains Fe in an amount of 8.0 mass % or less.
 4. The hot-dip galvanized steel sheet according to claim 1, wherein the hot-dip galvanized layer has a coating weight of 20 g/m² or more per surface.
 5. The hot-dip galvanized steel sheet according to claim 1, wherein the chemical composition of the base steel sheet further contains, in mass %, at least one selected from the group consisting of Nb: 0.040% or less, Ti: 0.030% or less, B: 0.0030% or less, Cr: 0.3% or less, Mo: 0.2% or less, V: 0.065% or less, and at least one selected from the group consisting of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr in a total amount of 0.1% or less.
 6. (canceled)
 7. A method of manufacturing a hot-dip galvanized steel sheet, comprising: a hot rolling process where a steel slab having the chemical composition as recited in claim 1 is subjected to hot rolling to obtain a hot-rolled steel sheet, a cold rolling process where the hot-rolled steel sheet is subjected to cold rolling to obtain a cold-rolled steel sheet, an annealing process where the cold-rolled steel sheet is heated to an annealing temperature, held at the annealing temperature, and then cooled, and then a coating treatment process where the cold-rolled steel sheet is subjected to hot-dip galvanizing treatment, wherein in the annealing process, an average heating rate in a temperature range from 500° C. to the annealing temperature is 1° C./s or higher and 7° C./s or lower, the annealing temperature is (A_(C1) point+50° C.) or higher and (A_(C3) point+20° C.) or lower, during the holding, a holding time is 1 second or longer and 40 seconds or shorter, during the holding, an atmosphere has a dew point of −20° C. or higher and 5° C. or lower, and a hydrogen concentration is 3 mass % or more and 20 mass % or less, an average cooling rate in a temperature range from the annealing temperature to a primary cooling stop temperature is 10° C./s or higher, the primary cooling stop temperature is 450° C. or higher and 600° C. or lower, a secondary cooling time is 20 seconds or longer and 100 seconds or shorter, and a secondary cooling stop temperature is 400° C. or higher and 500° C. or lower, and in the coating treatment process, a temperature of the cold-rolled steel sheet when it enters a coating bath is at least 10° C. higher than a coating bath temperature.
 8. The hot-dip galvanized steel sheet according to claim 2, wherein the hot-dip galvanized layer contains Fe in an amount of 8.0 mass % or less.
 9. The hot-dip galvanized steel sheet according to claim 2, wherein the hot-dip galvanized layer has a coating weight of 20 g/m² or more per surface.
 10. The hot-dip galvanized steel sheet according to claim 3, wherein the hot-dip galvanized layer has a coating weight of 20 g/m² or more per surface.
 11. The hot-dip galvanized steel sheet according to claim 8, wherein the hot-dip galvanized layer has a coating weight of 20 g/m² or more per surface.
 12. The hot-dip galvanized steel sheet according to claim 2, wherein the chemical composition of the base steel sheet further contains, in mass %, at least one selected from the group consisting of Nb: 0.040% or less, Ti: 0.030% or less, B: 0.0030% or less, Cr: 0.3% or less, Mo: 0.2% or less, V: 0.065% or less, and at least one selected from the group consisting of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr in a total amount of 0.1% or less.
 13. The hot-dip galvanized steel sheet according to claim 3, wherein the chemical composition of the base steel sheet further contains, in mass %, at least one selected from the group consisting of Nb: 0.040% or less, Ti: 0.030% or less, B: 0.0030% or less, Cr: 0.3% or less, Mo: 0.2% or less, V: 0.065% or less, and at least one selected from the group consisting of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr in a total amount of 0.1% or less.
 14. The hot-dip galvanized steel sheet according to claim 4, wherein the chemical composition of the base steel sheet further contains, in mass %, at least one selected from the group consisting of Nb: 0.040% or less, Ti: 0.030% or less, B: 0.0030% or less, Cr: 0.3% or less, Mo: 0.2% or less, V: 0.065% or less, and at least one selected from the group consisting of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr in a total amount of 0.1% or less.
 15. The hot-dip galvanized steel sheet according to claim 8, wherein the chemical composition of the base steel sheet further contains, in mass %, at least one selected from the group consisting of Nb: 0.040% or less, Ti: 0.030% or less, B: 0.0030% or less, Cr: 0.3% or less, Mo: 0.2% or less, V: 0.065% or less, and at least one selected from the group consisting of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr in a total amount of 0.1% or less.
 16. The hot-dip galvanized steel sheet according to claim 9, wherein the chemical composition of the base steel sheet further contains, in mass %, at least one selected from the group consisting of Nb: 0.040% or less, Ti: 0.030% or less, B: 0.0030% or less, Cr: 0.3% or less, Mo: 0.2% or less, V: 0.065% or less, and at least one selected from the group consisting of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr in a total amount of 0.1% or less.
 17. The hot-dip galvanized steel sheet according to claim 10, wherein the chemical composition of the base steel sheet further contains, in mass %, at least one selected from the group consisting of Nb: 0.040% or less, Ti: 0.030% or less, B: 0.0030% or less, Cr: 0.3% or less, Mo: 0.2% or less, V: 0.065% or less, and at least one selected from the group consisting of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr in a total amount of 0.1% or less.
 18. The hot-dip galvanized steel sheet according to claim 11, wherein the chemical composition of the base steel sheet further contains, in mass %, at least one selected from the group consisting of Nb: 0.040% or less, Ti: 0.030% or less, B: 0.0030% or less, Cr: 0.3% or less, Mo: 0.2% or less, V: 0.065% or less, and at least one selected from the group consisting of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr in a total amount of 0.1% or less.
 19. A method of manufacturing a hot-dip galvanized steel sheet, comprising: a hot rolling process where a steel slab having the chemical composition as recited in claim 5 is subjected to hot rolling to obtain a hot-rolled steel sheet, a cold rolling process where the hot-rolled steel sheet is subjected to cold rolling to obtain a cold-rolled steel sheet, an annealing process where the cold-rolled steel sheet is heated to an annealing temperature, held at the annealing temperature, and then cooled, and then a coating treatment process where the cold-rolled steel sheet is subjected to hot-dip galvanizing treatment, wherein in the annealing process, an average heating rate in a temperature range from 500° C. to the annealing temperature is 1° C./s or higher and 7° C./s or lower, the annealing temperature is (A_(C1) point+50° C.) or higher and (A_(C3) point+20° C.) or lower, during the holding, a holding time is 1 second or longer and 40 seconds or shorter, during the holding, an atmosphere has a dew point of −20° C. or higher and 5° C. or lower, and a hydrogen concentration is 3 mass % or more and 20 mass % or less, an average cooling rate in a temperature range from the annealing temperature to a primary cooling stop temperature is 10° C./s or higher, the primary cooling stop temperature is 450° C. or higher and 600° C. or lower, a secondary cooling time is 20 seconds or longer and 100 seconds or shorter, and a secondary cooling stop temperature is 400° C. or higher and 500° C. or lower, and in the coating treatment process, a temperature of the cold-rolled steel sheet when it enters a coating bath is at least 10° C. higher than a coating bath temperature. 